Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock

[Paolo Bonzini]

Il 19/09/2012 11:21, Avi Kivity ha scritto:
>> > I don't know if the front-end (device) lock should come before or after
>> > the back-end (e.g. netdev) lock in the hierarchy, but that's another story.
> I would say device -> backend.  It's natural if the backend is the timer
> subsystem, so extend it from there to the block and network layers.  Of
> course callbacks want it to be the other way round.

Yes, that's what I wasn't sure about.  In many cases I believe callbacks
can just release the backend lock.  It works for timers, for example:

    for(;;) {
        ts = clock->active_timers;
        if (!qemu_timer_expired_ns(ts, current_time)) {
            break;
        }
        /* remove timer from the list before calling the callback */
        clock->active_timers = ts->next;
        ts->next = NULL;

        /* run the callback (the timer list can be modified) */
-       ts->cb(ts->opaque);
+       cb = ts->cb;
+       opaque = ts->opaque;
+       unlock();
+       cb(opaque);
+       lock();
    }

(The hunch is that ts could be deleted exactly at the moment the
callback is unlocked.  This can be solved with ref/unref on the opaque
value, as you mention below).

Paolo

> We could solve the callback problem in the same way we're using for mmio
> callbacks.  When providing the callback, give the subsystem ->ref() and
> ->unref() methods.  As long as a callback is pending, the subsystem
> holds a reference.  Once the callback has been invoked or cancelled, the
> reference can be released.